The participants were randomly assigned to one of five groups. People in each of the first four groups watched a different video showing them a technique used to boost happiness, and were asked to carry out that technique every day for five days. The four techniques were forcing oneself to grin; performing random acts of kindness; expressing gratitude for something good in one's life; and thinking about a happy occurrence the day before.

The fifth group performed a "control" technique that involved simply thinking about the day before.

All five groups reported feeling more cheerful during the experiment. But the most significant improvement was seen in the group that thought about something positive that had happened to them the previous day. This technique led to a 15% increase in happiness relative to the control group. Expressing gratitude and smiling led to 8% and 6% improvements respectively.

Performing random acts of kindness was slightly less effective than simply thinking about the day before.

Professor Wiseman had hoped that the happiness generated by these mood-enhancing tricks would prove infectious, with the benefits extending beyond the people who watched the videos to lift the entire nation's mood (70% of the participants were in the UK).

In an attempt to test this hypothesis, he commissioned national surveys before and after the study, each asking a representative sample of 2,000 people to rate their cheerfulness. Sure enough, the results suggested that Britain's happiness had increased by 7 per cent by the end of the experiment.

"Obviously, it is impossible to say if this rise is due to the study," Wiseman said. "After all, it might be caused by many different factors, including world events or changes in the weather. However, we like to think that we played some role in helping put a smile on the nation's face."